This invention relates to an optical attenuator used for adjusting optical power incident on a detector in optical fiber transmission.
FIG. 6 shows a prior art optical attenuator. A reference numeral 1 denotes an incident fiber, 4 denotes an outgoing fiber, 5 denotes an optical fiber 6, 9 and 10 denote ferrules for holding fibers 1, 4 and 5 respectively, 13 denotes an end face of the incident fiber 1, 43 denotes an end face of the outgoing fiber 4, 51 and 52 denote end faces of the optical fiber 5.
The light propagated through the incident fiber 1 is emitted into space from the end face 13. A part of the light emitted into space is incident on the optical fiber 5 from the end face 51. Light propagated through optical fiber 5 is incident on the fiber 4 by way of the end faces 52 and 43, and is propagated through the fiber 4. It has been known that an attenuation in this case is determined by an air gap l between the end faces 13 and 51. For example, a structure of such prior art optical attenuator is disclosed in the Transactions of the IE/ICE, Symposium No. 2302, 1982.
However, the prior art optical attenuator shown has two defects pointed out as follows.
That is, since the fiber end faces 13 and 51 face opposite and in parallel relation to each other and are spaced apart across the air gap l, both the end faces 13 and 51 come to constitute, a Fabry-Perot interferometer to cause a multiple reflections therebetween. Thus, where a light with high coherence such as laser diode (hereinafter called LD) or the like is incident, a change in attenuation may arise due to temperature change or other cause. Moreover, since the light reflected on both fiber end faces 13, 51 returns to a light source, the optical attenuator is hard to use on a system (using LD for example) on which the returning reflected light may adversely impact.